Thermal expansion of select M_n+1AX_n(M=early transition metal, A=A group element, X=C or N) phases measured by high temperature x-ray diffraction and dilatometry

Herein we report on a systematic investigation of the thermal expansion of select M_n+1AX_n phases. The bulk dilatometric thermal expansion coefficient αdil was measured in the 25-1200 °C temperature range and the thermal expansion of more than 15 of these phases was studied by x-ray diffraction in the 25-800 °C temperature range. The coefficient of thermal expansion for the a axis αa ranged between (2.9±0.1) × 10^-6 ° C^-1(Nb₂ AsC) and (12.9±0.1) × 10^-6 ° C^-1 (Cr₂ GeC) while the coefficient for the c axis (αc) ranged between (6.4±0.2) × 10^-6 ° C^-1 (Ta₂ AlC) and (17.6±0.2) × 10^-6 ° C^-1 (Cr ₂ GeC). Weak anisotropy in the thermal expansion was seen in most phases, with the largest value of αc / αa belonging to Nb2 AsC. The Grüneisen parameters along the a and c directions were calculated from ab initio values for the elastic compliances and were relatively isotropic. A good correlation was found between the thermal expansion anisotropy and the elastic constant c₁₃ and we conclude that the anisotropy in thermal expansion is related to the bonding between the M-A elements.